(1) Field of the Invention
The present invention relates to a wavelength selective switch that can select a desired wavelength channel from a wavelength division multiplexed (WDM) light applied to an input port, and output it to a target output port. In particular, it relates to a wavelength selective switch that has a function for monitoring the optical power output from a plurality of output ports.
(2) Description of Related Art
The recent rapid spread of high speed access networks that have bandwidths of approximately several Mbit/s to 100 Mbit/s, such as FTTH (Fiber to The Home), ADSL (Asymmetric Digital Subscriber Line), and the like, has created an environment in which broad bandwidth internet services can be received. In order to deal with the increase in communication demand under such conditions, mass storage optical communication systems in which wavelength division multiplexing (WDM) techniques are used, are being built into backbone networks (core networks).
In the connections between the above-described core networks and Metropolitan Area Networks that users access directly, there are concerns about the occurrence of bandwidth bottle necks due to limitations in electrical switching capability. Therefore, it is effective to provide new optical switching nodes in metropolitan regions that have become bandpass bottle necks, and build new photonic network architectures for connecting between core networks and metropolitan area networks directly in the optical domain without the intervention of electrical switches. Hence, a lot of effort is being put into research and development to realize this.
As one type of optical switch module to be used as a node for connecting core networks and metropolitan area networks, for example a wavelength selective switch (WSS) is known (for example, refer to U.S. Pat. No. 6,549,699, Japanese Unexamined Patent Publication No. 2000-347065, and Japanese Unexamined Patent Publication No. 2001-330865)
FIG. 15 is a perspective view showing a structural example of a conventional wavelength selective switch.
In FIG. 15, a wavelength selective switch 100 comprises a fiber collimator array 110, a diffraction grating 101, a lens 102, a mirror array 103, and a ¼ wave plate 104. The fiber collimator array 110 has N (≧3) fiber collimators placed side by side in one direction, and constitutes one input port 110-1 and a plurality of output ports 110-2 to 110-N. After a WDM light coming out from the input port 110-1 is separated by the diffraction grating 101 in directions with different angles depending on their wavelengths, lights of each wavelength (referred to hereunder as wavelength channel) are focused onto different locations by the lens 102. At the location where the wavelength channels are focused, a mirror array 103 having a plurality of MEMS mirrors corresponding to the number of channels is placed. The MEMS mirrors are micro mirrors formed by using a micromachining (Micro Electro Mechanical System: MEMS) technique, and the angles of each of their reflecting surfaces can be controlled in accordance with a drive signal. Each of the wavelength channels that reach the mirror array 103 is reflected by a corresponding MEMS mirror, and returned in a direction according to the angle of the respective reflecting surfaces. At this time, by being controlled to a predetermined angle corresponding to the location of any one of the output ports, which is set in the output address of the wavelength channel to be injected, the wavelength channel returned by each of the MEMS mirrors passes through the lens 102, the ¼ wavelength plate 104, and the diffraction grating 101 in order, and is guided to a respective target output port.
FIG. 16 is a perspective view showing another structural example of a conventional wavelength selective switch. The wavelength selective switch 100′ is different from the structural example shown in FIG. 15 in that a transmission type diffraction grating 101′ is used. The other structures are the same as in the case of FIG. 15.
The conventional wavelength selective switches 100 and 100′ as described above have a wavelength selective function whereby, with respect to a plurality of wavelength channels contained in an input WDM light, a desired wavelength channel can be selected and guided to a target output port by controlling the angles of the reflecting surfaces of each of the MEMS mirrors on the mirror array 103. Furthermore, by reversing the relationship between input and output, it is also possible to guide a WDM light containing different wavelength channels applied to a plurality of input ports, to one output port.
Regarding the conventional wavelength selective switches described above, it is important that the reflecting surface of each of the MEMS mirrors on the mirror array is controlled to be a predetermined angle. Therefore, a construction is proposed in which, as shown in FIG. 17 for example, an optical tap 120 is provided in each output fiber of the wavelength selective switch to branch part of the output light. The branched light from each of the optical taps is then applied to a spectrum monitor 121 to monitor the power of each of the wavelength channels, and the angles of the reflecting surfaces of corresponding MEMS mirrors on the mirror array are feedback controlled according to a control signal output from a processing unit 122 based on the monitored result (for example, refer to U.S. Pat. No. 6,549,699).
However, in the monitoring structure of an output light in the conventional wavelength selective switch as shown in the example of FIG. 17, it is necessary to provide individual optical taps to each of a plurality of output fibers. Therefore, there are problems in that the module size is increased due to the increase in the number of optical parts, and in that the cost increases.
As another structure for monitoring an output light in a conventional wavelength selective switch, as shown in FIG. 18 for example, it is also possible that an optical branching device, such as a prism or the like, is provided on the optical path between the mirror array and each of the output ports in the wavelength selective switch, and light branched by the optical branching device is received by a PD array, and the power of the light guided to each output port is monitored. However, in this construction, since any influence such as a shift in the optical axis of the optical parts after passing through the optical branching device is not contained in the optical power monitored by the PD array, there is a possibility that the power of the light physically guided to each output port, and the monitored power in the PD array do not match.